This application is related to U.S. patent application Ser. No. 09/684,259, entitled xe2x80x9cA Measurement System And Method of Determining Characteristics Associated With a Waveform Having Distortion Associated Therewith,xe2x80x9d by Akshay Aggarwal, et al., filed on Oct. 6, 2000.
The present invention is directed, in general, to electronic circuits and, more specifically, to a distortion precompensator and method of compensating for distortion in a transmission medium and a transmitter employing the same.
A continuing trend in telecommunications networks is the higher data rate associated with the transfer of data. To achieve such high speeds of data transfer, conventional telecommunications networks include output buffers that produce signal pulses, or waveforms, for transmission over a telecommunications line. The output buffers are generally employed in transmission devices to drive a signal to a destination location such as a receiver. Such signals can traverse, via the transmitter, a transmission medium (e.g., a board trace, a cable or other electrically conductive medium) before arriving at the destination location.
Complications, however, arise in connection with the transmission of information across the transmission medium. For instance, mismatches in impedance at interfaces located within the telecommunication network can cause distortion in the transmission medium in the form of reflections of the incident wave, which may then travel back to the source of information. This is particularly noticeable when signals travel through the interface of the board trace and the cable having different impedances. In such situations, an initial reflection wave may be generated back to the transmitting output buffer. If the transmitting output buffer has an impedance which is different from that of the board trace, a subsequent reflection wave may be generated, which then travels back to the destination location. Due to this iterative behavior, a xe2x80x9cstaircasexe2x80x9d effect may be exhibited in the waveform at the output. Also, the staircase effect is usually pattern dependent, and hence, may potentially induce data dependent jitter. Moreover, with the current trend of very high speeds of data transfer, where the output rise and fall times are much faster with respect to the cable lengths, the problem of reflection becomes even more troublesome.
To overcome the xe2x80x9cstaircasexe2x80x9d effect caused by the mismatches in impedance, telecommunications network designers currently attempt to match the output impedance of the output buffer with the other impedance characteristics associated with the telecommunications network, including the characteristic impedance of the board trace. Theoretically, if the impedances of the output buffer and the board trace match, any reflection caused by the impedance mismatch between the board trace and the cable should be fully absorbed. In such situations, and assuming that no other reflections are generated in other portions of the transmission medium, a transmitted signal should attain a clean response. While in theory the principle is sound, realistically it is very difficult to match the impedances of the output buffer and the board trace. Moreover, the techniques presently available to match the impedances are expensive and time consuming to implement.
Accordingly, what is needed in the art is a network that compensates for distortion in a transmission medium, that overcomes the deficiencies of the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides a distortion precompensator and method of compensating for distortion in a transmission medium and a transmitter employing the same. In one embodiment, the distortion precompensator includes a controller, associated with a transmitter, that employs a predetermined precompensation signal. The distortion precompensator also includes an injector that injects a predetermined precompensation value as a function of the precompensation signal into a waveform propagating along the transmission medium, thereby substantially compensating for distortion associated therewith.
The present invention introduces, in one aspect, a distortion precompensator that compensates for distortion in a transmission medium of a telecommunications network. The injector of the distortion precompensator modulates the precompensation signal to account for the distortion in the transmission medium. Thus, the distortion precompensator can counteract, for instance, impedance mismatches between segments of the transmission medium or telecommunications network. Typically, the distortion precompensator is associated with a transmitter of a telecommunications network. In an embodiment to be illustrated and described, the controller includes cascading flip-flops and a multiplexer, and the injector includes at least one controllable switch and a programmable current source.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.